In a species of sweet pea (Lathyrus odoratus), flower colour and pollen grain shape are determined by two genes located on the same autosome. A dihybrid plant heterozygous for both characteristics ($PL/pl$) is test-crossed with a double homozygous recessive plant ($pl/pl$).

The phenotypes of the resulting 1000 offspring are recorded as follows:

• Purple flowers, long pollen: 415
• Purple flowers, round pollen: 85
• Red flowers, long pollen: 75
• Red flowers, round pollen: 425

What is the recombination frequency between the locus for flower colour and the locus for pollen shape?